Post-Planck dark energy constraints

We constrain plausible dark energy models using the recently published cosmic microwave background (CMB) temperature anisotropy data from Planck together with WMAP9 low-l polarization data and the data from low redshift surveys. To circumvent the limitations of any particular equation of state toward describing all existing dark energy models, we work with three different equations of state covering a wider class of dark energy models and hence provide more robust and generic constraints on the dark energy behavior. We show that a possible tension exists between constraints from CMB and non-CMB observations when one allows for both phantom and nonphantom behavior for the dark energy. Further, we reconstruct the equation of state of dark energy as a function of redshift using the combined CMB and non-CMB data and show that cosmological constant behavior is disallowed at the 68.3% confidence level. A fully nonphantom history is also disallowed at the 68.3% confidence level, and a considerable fine-tuning is also needed to keep it inside the 95.5% confidence limit. This result might motivate one to construct phantom models for dark energy, which may be achievable in the presence of higher derivative operators as in string theory. However, for a theoretical model that allows only nonphantom behavior, both CMB and non-CMB data sets agree on the dark energy constraint with the mean equation of state being very close to the cosmological constant.